The method conventionally used for the growing of crystals with the general structure A3B5C12 (e.g. Y3Al5O12 (YAG), Lu3Al5O12 (LuAG) or (Lu,Y)3Al5O12 (LuYAG)), with diameters of up to 80 mm and doped with various elements, is the Czochralski method. At the present time these crystals are intensively studied as prospective materials for very high efficiency solid state laser technologies, but have other uses such as in scintillators, optical elements, components for instrument or lighting engineering, and in jewelry making. Hence it is most desirable to prepare high-quality crystals with larger diameters. Such materials with diameters of 200 mm or even larger, and in optical quality have not yet been prepared. Growing of crystals by the Czochralski method ordinarily takes place in an iridium crucible. The melting point of the YAG crystal is 1950° C., the melting point of LuAG is 2050° C. and the melting point of iridium is 2 440° C., while the maximum critical temperature Tcrit for using an iridium crucible without causing any damage thereto is 2 300° C. The growing of large diameter YAG and LuAG crystals by the Czochralski method from iridium crucibles is very difficult due to the large radial temperature gradient (the difference in melt temperature at the crucible wall and in the middle thereof) which is essential for the preparation of optical-quality crystals. The growth zone of the crystal is separated from the tempering zone by a crucible lid. The crystal is pulled above the crucible lid; heat removal is resolved by pulling the crystal above the crucible lid where the tempering temperature is substantially lower than in the growth zone. Because of that, in order to grow large crystals it is necessary to modify the original Czochralski method or to prepare a different method.
An example of such modified technology for growing YAG, LuAG and GGG crystals with diameters above 80 mm by the Czochralski method is U.S. Pat. No. 7,476,274. It is possible to thus grow crystals only with very careful control of the phase interface and the maintenance thereof planar (not growing into the melt); and in order to remove the steep temperature gradient at the transition between the growth and tempering zones additional heating (an after-heater) is used. The process is controlled via optical process control and a constant growth during the process is maintained by regulation of the rotation speed. Such process complexity has negative effect on the yield of these crystals, and achieving the required large sizes still remains difficult.
Patent CZ 300 631 B6 describes the preparation of crystals for scintillation detectors and solid state lasers doped with rare earths.
Patent CZ 248913 describes the growing of single crystals from single and even multi-component oxide melts with maximum utilization of the melt contained within the crucible. This is achieved through temperature regime regulation and by defining the position of the maximum temperature isotherm in the crucible using the temperature gradient in the direction of the top rim of the crucible. The process is completed by the crystallizing of the material in the crucible. A particular disadvantage of this method is the fact that it is not possible to incorporate the same quantity of dopant within the crystal as is the concentration thereof in the melt; hence, the result is an inhomogeneous crystal. The aforementioned paper is not concerned with the issue of optical homogeneity of the crystal at the level currently required in view of the technical applications thereof.
Paper JP 6183877 A describes the growing of crystals by the Czochralski method wherein the temperature at the phase interface is controlled using an arrangement of thermocouples.
Apart from the aforementioned patent, large diameter yttrium and rare earth aluminate crystals were up until now prepared only using the HEM (Heat Exchange Method) and TGT (Temperature Gradient Technique) methods, both of which are very equipment-intensive.
The HEM method (Khattak, C. P. and F. Schmid, Growth of the world's largest sapphire crystals. Journal of Crystal Growth, 225(2001)572) uses a large-diameter molybdenum crucible which is placed within the apparatus in a support that is in contact with a relatively small-diameter heat exchanger end. An oriented seed crystal is placed at the bottom of the crucible at the point of contact with the heat exchanger and the crucible is loaded with melt charge. The apparatus is closed and evacuated and the temperature of the charge is gradually increased. Before melting down the charge, helium gas is forced through the heat exchanger and washes over the crucible bottom in the position where the seed crystal is located, so that the seed crystal does not melt down completely but there is only partial meltdown thereof. Crystal growth is initiated and maintained by further temperature reduction in the heat exchanger. Crystallisation starts on the partially melted seed crystal at the bottom of the crucible. During the growth stage cooler material stays near the bottom of the crucible and hotter material is in the upper part thereof, which stabilizes the temperature gradient and minimises convection in the melt. During growth the growing crystal remains submerged under the surface of the melt at all times and is thus protected against mechanical and temperature perturbations by the surrounding melt. This stabilises the melt-crystal phase interface which, in this case, is curved and hence it is not necessary to equilibrate these asymmetries by rotating the crucible or crystal. Complete solidification of the charge is achieved by controlled reduction of the furnace heat exchanger temperature. The last portion of the material to crystallise is located at the crucible wall. The HEM method has the primary disadvantage of high costs in a process using expensive helium.
The difference between the HEM and TGT methods is that TGT does not use helium (nor any other gas) as the heat transfer medium and the temperature gradient is created by suitable heating element geometry and by using water-cooled graphite electrodes. The seed crystal is inserted into a narrowed cylindrical or conical feature in the crucible bottom, which in turn is placed onto a water-cooled metal (molybdenum) support. Similar crucible design is also used in the Bridgman-Stockbarger method. For the reasons stated hereinabove, using conical-shaped crucibles is preferable for growing crystals. The TGT method was patented in 1985. However, garnet structure crystals (for now YAG) still do not exhibit sufficient quality parameters utilizable in the manufacture of demanding optical elements (Yang, X. B., et al., Growth of large-sized Ce:Y3Al5O12 (Ce:YAG) scintillation crystal by the temperature gradient technique (TGT). Journal of Crystal Growth, 311(2009)3692). The TGT method is also described in Chinese patent 101705516 A.
Large size aluminates are also grown by the Bagdasarov method (also called Horizontal Directed Crystallisation). Crystals are grown from the melt in a boat-shaped crucible which is pulled in the horizontal direction across a temperature gradient. The grown material is melted by passing the crucible through a heating zone wherein the crystalline phase is created in a suitably selected temperature gradient. In order to obtain a crystal with an exact orientation, a seed with the desired orientation is placed in the narrow part of the boat. These crystals have a rectangular (not round) shape and their optical properties are generally regarded as being inadequate.
Several other methods are used to grow large diameter crystals. Worth mentioning are primarily the following: EFG, Stepanov, Stockbarger, Bridgman, etc. However, these methods are not used to grow the aforementioned large-sized crystals, whether in terms of quality or even the ability to achieve garnet structure crystals with diameters over 100 mm.
Neither is the Kyropoulos method of preparation used to grow large YAG crystals. This method is primarily employed to grow large sapphire and titanium-doped sapphire single crystals. Out of other materials, the following are grown using this method: CsI, CaF2, CsB3O5, LiF, KYb(WO4)2, NaCl, KCl, KBr and some large diameter semiconductor crystals such as InP, GaAs, and ZnTe.
The method was first used in 1926 for the preparation of single crystals (Z. Anorg. Chem. 154(1926)308). This method of single crystal preparation can be implemented with resistance heating as well as with induction heating of the crucible, with arbitrary growth atmosphere compositions (including vacuum) and arbitrary crucible materials. The principle of the Kyropoulos method partly proceeds from the Czochralski method of single crystal preparation (Z. Physik. Chem. 92(1917)219). The apparatus for the Kyropoulos method is similar to that for the Czochralski method. If we compare single crystal preparation techniques by the Czochralski and Kyropoulos methods, it is evident that the Czochralski method is suitable for the preparation of longer, smaller-diameter crystals, and, on the contrary, in the Kyropoulos method the growing crystal is not pulled out of the melt via the seed crystal (Czochralski), but growth is controlled by the removal of heat through the seed crystal and by reducing the melt temperature such that the growth isotherm, corresponding to the melting point of the grown crystal, proceeds into the melt and crystal growth occurs under the surface of the melt. Upon melting the charge, the homogenisation thereof and upon setting the growth temperature, an oriented seed crystal of the required material with a square or circular cross-section and rotating at a low speed (2-5 rpm) is submerged into the melt to the geometric centre of the crucible. For the Czochralski method, it is characteristic that the crystal diameter is at most 0.6 times the crucible diameter.
Another method for growing large crystals is the SAPMAC (Sapphire Growth Technique with Micro-Pulling and Shoulder Expending at Cooled Centre) method (Cryst. Res. Technol. 8(2007)751. The SAPMAC method is based on pulling a sapphire crystal out of the melt contained in a molybdenum crucible by introducing a cooled seed crystal, widening the profile to the required size, and a combination of slow pulling of the crystal and the slow ingrowing thereof into the melt.
For growing crystals in an oxidizing atmosphere (nitrogen+oxygen mixture), growing takes place in a furnace with induction heating in an iridium crucible. In the case of using a reducing atmosphere (argon+hydrogen mixture), growing takes place in a resistance furnace in a molybdenum or tungsten crucible. The decisive factor for using either type of growing atmosphere when growing doped YAG or LuAG crystals is, apart from other things, the required oxidation state of the dopant. In the case of growing YAG crystals: Yb3+, LuAG:Yb3+ it is necessary to use an oxidizing atmosphere (nitrogen+oxygen mixture) in order to prevent the reduction of Yb3+→Yb2+, and growing must take place in an Ir crucible. On the other hand, in the case of YAG:Ce crystals where the presence of Ce3+ is desirable, a molybdenum or a tungsten crucible and a reducing atmosphere are preferred.
Large garnet structure crystals (YAG, LuAG, GGG doped with oxides of cerium, praseodymium, neodymium, ytterbium, samarium, holmium, dysprosium, erbium, terbium and thulium, but also of vanadium, manganese and titanium) are finding utilization in a number of applications whereof the primary ones include scintillation and imaging (e.g. medical, safety, non-destructive testing, instrumentation), lighting (wafers or conventional lighting elements in combination with LED), jewelry making or laser (high-efficiency solid state lasers).
As is apparent from the foregoing overview, at present there is no satisfactory method for the preparation of these materials in the requisite optical quality, price and size.